The broad, long-term goal of this application is to investigate the cellular and molecular mechanisms involved in establishing the precise axon connections between retinal photoreceptor neurons and their target neurons in the brain of Drosophila. The specific aims are: (1) to use genetic mosaic analysis to determine which cell populations in the visual system require the function of a particular gene; (2) to use cell-type specific markers to determine which cell populations are present and differentiate normally in the mutant animals; (3) to characterize at the molecular level genes selected on the basis of the analyses described in aims 1 and 2. Mosaic analysis will be used to determine cell populations that require the function of a particular gene, and the generation of mosaic animals will use the yeast FLP recombinase/FLP system. Antibodies recognizing specific cell types will be used to determine cell identity in mutant animals. In addition, mutant strains with specific cell populations marked with a reporter gene lacZ will be created and different cell types determined by an anti-beta-galactosidase antibody. The analysis will include confocal microscopy. The genes identified in these analyses will be characterized by molecular cloning, where Drosophila cDNA libraries will be screened using probes from plasmid rescue, and full-length sequences of mutant genes obtained. Finally, phenotype-rescuing analysis through mRNA injection will be performed to confirm the rescuing ability of a cloned gene.